The aim of the proposed studies is to clarify and quantify the effects of hormones during early life on the development of sexual and related behavior patterns in adult mammals. A marsupial mammal will be used since marsupials are born after brief gestation in a developmentally immature "fetal" condition. This marsupial model system permits direct visualization, quantification and manipulation of a mammalian prenatal (prenatal in a traditional mammalian model and postnatal in a marsupial mammalian model) environment. In the first group of studies, sexual and sexually dimorphic behavior patterns of a marsupial Monodelphis domestica, the gray short tailed opossum, will be correlated with changes in vaginal cytology and blood hormone levels. The hormone environment of neonates will then be quantified and manipulated and the effects of these manipulations on the previously quantified adult sexual and sexually dimorphic behavior patterns will be observed. Alterations in the prenatal environment of humans by hormones which affect adult behavior when applied prenatally such as progestins and corticosteroids and commonly used drugs such as alcohol, marijuana and diazepam are probable. Development of the unique marsupial model system proposed here will provide access to the heretofore inaccessable mammalian prenatal environment. As such, studies utilizing this system will further our knowledge about the effects of alterations in this environment on behavioral development in humans.